In a conventional prior art carburetor 20 as best illustrated in FIG. 1, a fuel-and-air mixing passage 22 extends through a carburetor body 24 providing a fuel-and-air mixture to the crankcase of a two cycle combustion engine or to an intake manifold of a four cycle engine. A throttle valve (not shown) orientated in a downstream region of the fuel-and-air mixing passage 22 controls the fuel-and-air mixture flow, which in-part controls the speed and power of the operating engine. Similarly, a choke valve 26 is orientated in an upstream region 28 of the mixing passage 22 and controls the amount of air flow through a venturi of the mixing passage 22 which is located between the throttle and choke valves. A main fuel feed tube communicates transversely with the mixing passage 22 at the venturi to flow liquid fuel from a fuel chamber or bowl and into the passage to mix with the flowing air. The amount of liquid fuel emitted is dependent upon the amount of vacuum created at the venturi by the operating engine and positioning of the valves.
During cold engine starts, a rich mixture of fuel-and-air is needed. To produce the rich mixture, the throttle valve is substantially open exposing the fuel feed tube or nozzle to the vacuum of the cranking engine, and the choke valve 26 is generally closed to reduce air flow. When the engine is idling at operating temperature, the throttle valve is substantially closed (typically slightly open or closed with a notch or hole therein permitting sufficient mixture flow to support engine idling or low load operation) producing a high vacuum condition downstream of the throttle valve and the choke valve is open. The closed throttle valve reduces air flow through the venturi which reduces liquid fuel flow emitted from the fuel feed tube. The resulting low fuel-and-air mixture flow rate coincides with the needs of the engine running at idle, or low speed or low load. During steady, high speed and full load engine operation, the throttle and choke valves are generally wide open causing a high air flow rate through the venturi which produces a high vacuum for emitting a commensurate amount of fuel through the main fuel feed tube.
For smooth engine acceleration from idle, however, and generally as the throttle valve is opening, the engine requires a richer mixture of fuel-and-air than at hot idle or high speed light load. A diaphragm-type acceleration pump 30 supplies this additional amount of fuel by sensing vacuum pressure changes downstream of the throttle valve. When the engine is idling and the throttle valve is substantially closed, the vacuum pressure downstream of the substantially closed throttle valve is generally high. For example, in a typical four cycle engine application the vacuum can be about ten inches of mercury. An external conduit or hose 32 of the pump 30 communicates this vacuum with a vacuum chamber defined in part by the diaphragm of the pump. When the vacuum is high (i.e. ten inches of mercury), the diaphragm is flexed into the vacuum chamber thus maximizing the volume of a supplemental fuel chamber defined in-part by an opposite side of the diaphragm. When the engine is accelerating, the throttle valve is opening causing the vacuum pressure to drop, for example, down to about one inch of mercury. This drop in vacuum is sensed by the pump 30 through the hose 32 and joining tube fittings 34, 36 causing the diaphragm via the assistance of a compression spring to move into the supplemental fuel chamber which pushes the supplemental fuel through a discharge hose 38 coupled to joining connector tube fittings 40, 42 by clamps 44, 46, and into the fuel-and-air mixing passage 22 immediately upstream of the venturi.
As shown in the illustration of FIG. 1, the acceleration fuel pump 30 is formed into a float bowl 48 of the carburetor 20 in such a way that the fuel chamber of the pump is defined between the diaphragm and the float bowl 48. The vacuum chamber is defined between the diaphragm and a pump cover 50 attached to the float bowl by screws 52. Unfortunately, and even with the integration of the pump 30 into the float bowl 48, the carburetor 20 still requires a plurality of external parts to communicate the acceleration pump with the fuel-and-air mixing passage 22. For instance, the fuel discharge channel and the vacuum-sensing channel require the fittings, hoses, and clamps previously described. The supply and assembly of these parts is costly, leads to maintenance concerns and each connection is a source for a potential leak. Yet further, many portions of the various channels require drilling passages into the body 24 of the carburetor 20 at compound angles which also require various plugs to seal an open end of the passages.